This fixes one bug and several deficiencies in the previous
implementation, which was based on read effects. One caveat
about the current version is that we treat all type variables
in a polymorphic type as visible (= non-ghost) even if they
only occur in the ghost fields. For example, the following
code is rejected:

  type strange 'a = { ghost contents : 'a }

  let test () =
    let x = { contents = ref 0 } in
    x.contents := 42

Even though the field [contents] is ghost and cannot be reached
from the real-world code, the type of x is [strange (ref int)]
and the region of the reference is considered to be visible.
Thus the last assignment is seen as an illicit ghost write.

On the other hand,

  type strange 'a = { ghost contents : ref 'a }

and

  type strange 'a = { ghost mutable contents : 'a }

permit ghost writes into [contents] without problem.
Ghost writes into ghost variables are also okay.